What makes you think Buckbeard/The Vandenreich Leader is an Ex-Shinigami? You can't say that for sure.
I don't think he was a Shinigami. We know that there is a complicated conflict between Soul Society and the Quincy and that the Shinigami wiped out nearly all Quincies to prevent Soul Society spilling over in the mortal world which would mean the end of thd world. We've seen how great the animosity between Quincy and Shinigami is/can be when Uryuu was introduced. It'd certainly hurt the pride of a Quincy to bow down to a Shinigami. Also Shinigami don't have the power over Reishi/spirit particles as Quincy have – The Leader manipulated them to construct a stairway to the chamber where Harribel is kept out of the particles from his throne.
"Shinigami use the spiritual power created from their soul in conjunction with their Zanpakutos whereas Quincy use the particles around them" (Source: Bleach-Wiki, Article "Qunicy": [link] )
Given the fact that him and yama duked it out over a thousand years ago and he still didn't age that much (he prob aged a few years or so we can safely deduce that he's a shinigami.
This guy was prob part of squad zero or at least had some connection with the royal society and yes the feud between shinigami and quincies has been going on for more than a thousand years in which the shinigami wiped out almost all of the quincies.
However, as we've clearly seen being a 'quincy' doesn't require you to be human as 'quincy' is a teaching not a race given the fact that even arrancars can become one.
So if say, a shinigami were to learn these teachings then it wouldn't be so far fetched that he'd be able to control and manipulate reishi as he wants.